Due to an afliction (cyst) on my low hand little finger, I cannot use the offset bottom hole. There is a "tunable" chanter by LIS on the for sale site. It is more complicated and expensive than I need and I already have a very nice chanter that has a silvered sole.
1. Does anyone know if there is a chanter maker who can take an existing chanter and make the bottom hole rotate on a tenon like the foot joint on a flute? I would rather not drill a new hole as it would require filling the existing hole and the crook in my little finger changes over time and with environmental conditions so even drilling a new hole might not solve the problem entirely. Corrective surgery is not in the cards. I cannot find a surgeon who wants to remove the cyst as it is not causing me pain and it is wrapped around a nerve making the downside of an operative error very possible and irreversible.
2. Looking at youtube videos of players, most of them use the bottom hand flat (fingers straight), the same as highland pipers. Why are the bottom holes offset when the apparent overriding technique would preclude that?

First.
The bottom holes are offset on a lot of woodwind instruments of all kinds, coming from a lot of cultures/times. The reason is simple. It is the rest of the fingers that are used in a bent position. Point is, the "piper's grip" is not by any means "normal". In fact, it gets in the way of a lot of more sophisticated, fast techniques that are normal with, once again, a lot of woodwinds. Fingertips are simply more flexible. That's why, in a word.
Now, as to the point. The way to do what you asked ( taking an existing chanter, and making the bottom hole revolve) is not impossible, but is rather a pain. What it involves is taking the chanter, putting it on the lathe between two centres, praying that it has not gone out of true shape too much (a rather futile prayer, to tall the truth); then cutting the bottom part off half-way between the two last holes (still on the lathe, that is.). Now, next step is putting the two parts back on the lathe between the two centres, and turning a tenon on each, just a fraction thinner than a (yet to be explained) socket's inside diameter. The socket in this case should be a brass (silver, gold, whatever) tube, about 15-20 mm long, the outside diameter of which is more-or-less equal to the outside diameter of the chanter at that point. The tenons on both sides should be turned with a small recess to take cotton (waxed cotton, that is) binding. These both fit tightly into the brass socket. The length of the tenons put together should equal the length of the socket.In theory any competent turner should be able to do it, but in reality I'd prefer to trust someone who has experience with woodwind instruments.
Where are you located?

Thank you for your response. I have a flute and bagpipe maker in my neighbourhood, Casey Burns out of Kingston, WA, USA. I'm confident he could do it. You brought up an interesting point...quick movements with the fingers. My GH bagpipe instructor said the "piper's grip" (flat finger position) was to facilitate the quick movements (e.g. birl & throw on d). I take it from your comment above that there are movements done on the gaitas that are better performed or facilitated with bent fingers. I've not found a gaita instructor locally so other than looking at youtube videos my experience is limited. As I mentioned above, most of the videos I've seen show the players using a flat finger lower hand technique. Since I also play the Scottish small pipes, I was hoping to keep the playing technique between the two similar if possible.

It's not the gaitas specifically. Thing is, these days most woodwinds come with a bank of keywork stuck on them. You simply cannot play them with a piper's grip. But even without the keys, fingertips are simply more sensitive than the middle joints. The advantage of piper's grip lies in the fact that this vary fact can be a plus. If you are piping in driving sleet/snow, well, fingertips will fail you. The middle joints won't, at least not as readily. At the same time, most woodwind players prefer comfy, indoor, heated environments. It takes a Scot to prefer the bracing hurricane... (OK, just kidding, of course.)

Fortunately, my Scot infusion is very small...as far as I have been able to tell, one slip back in the early 1600's. As such, I don't think I'll be taking the small pipes out in the driving snow and sleet. The Dane and Russian in me, however, might counteract what I want to regard as sanity with respect to weather activities. I like the small pipes because I can play them inside without damaging my hearing which is deteriorating with age and abuse as it is. I just might have both chanters, Gaita and Small pipe, modified so I can rotate the bottom holes to accomodate whatever angle my crooked finger happens to take that time of day.
Thank you for your help and humor!